1. Field of the Invention
The present invention relates to semiconductor devices, and in particular to devices having hole mobility enhancement and/or density of state reduction through the use of one-dimensional or two-dimensional p-type quantum wells.
2. Description of the Prior Art
In semiconductors, there are both light and heavy holes in the valence bands. They are usually degenerate in energy at the point of the k-space in most bulk materials. The density of states of heavy holes is larger than that of light holes, while the mobility of heavy holes is lower than that of light holes. The low mobility of heavy holes leads to the low speed of conventional p-channel devices in which the conduction is primarily carried out by heavy holes. Consequently, the speed of conventional complementary structures comprising a pair of n-channel and p-channel devices is also limited. This has been a major difficulty for complementary circuits since their invention. In addition, the larger density Of state associated with large hole effective mass has been partially responsible t the problem of large threshold current in modern optoelectronic devices.
Along with the rapid advance of high speed compound semiconductor devices, fast p-channel devices have become increasingly important. To compete with the Si CMOS technology, a great deal of effort has been devoted to the development of III-V compound complementary devices in the last decade. One of the major problems with the III-V compound complementary devices is that while very high speed n-channel devices have been developed, the p-channel devices to date are relatively slow mainly because of the heavy hole effective mass. This means that the speed of a III-V complementary device is limited to the speed of its p-channel device which is approximately ten times slower than the corresponding n-channel device. Recently, some progress toward III-V complementary structures has been made and complementary undoped heteroinsulator FETs utilizing two-dimensional (2-D) electron and hole gases has been successfully demonstrated. However, the mobility and velocity of 2-D hole gas in such devices are still much smaller than that of 2-D electron gas.
The tetrahedral symmetry in a zinc-blende semiconductors may be broken by strain resulting in splitting in hole subbands at the point and a mass in the lowest hole subband lighter than the heavy hole mass. In principle, the performance of p-type devices may, therefore, be greatly enhanced through utilizing such light hole states. Recently, this important observation has stimulated a great deal of interest in possible device applications.